The field of the invention is automatic chokes for internal combustion engines, and more specifically, automatic chokes for small engines having one or two cylinders.
The use of automatic chokes on automotive engines is well known. Typically, a choke valve, or "butterfly", is mounted in the air intake passage of the carburetor and means is provided to automatically orient this valve to provide the desired air-fuel mixture. For example, the choke valve is usually substantially closed while the engine is being started to provide a rich air-fuel mixture. As the engine warms up, the valve is automatically opened to provide a leaner, cleaner burning mixture.
With the recent emphasis on reducing the emissions from automotive engines a number of temperature responsive automatic choke assemblies have been proposed. As disclosed in U.S. Pat. No. 3,752,133, for example, temperature responsive bimetallic coils are used in some of these chokes assemblies to open the choke valve as the engine warms up. Heat directly from the engine may be used to operate the bimetallic coil and in some instances this may be supplemented by a heating element which accelerates opening. Or, as disclosed in U.S. Pat. No. 3,818,881, the bimetallic coil may itself be used as the heating element by passing electric current through it.
A number of unique problems are presented when starting small engines and engines which are controlled by speed governors. When an automatic engine is started, a fast idle cam on the carburetor is engaged to automatically open the throttle a preset amount. A vacuum piston connected to the manifold is operated when the engine starts and this tends to pull the choke valve open against the resistance of the bimetallic coil. As the engine warms up, the resistance of the coil gradually decreases and the choke valve opens. Such a system does not work on small engines, or on engines which are started with the throttle wide open, since a uniform manifold pressure cannot be maintained to provide a force which will gradually open the choke valve against the resistance of the bimetallic coil. Also, on governor controlled engines which are started with the throttle wide open, some choke is required for starting even when the engine is hot.